Apparatus for reducing shock and overpressure

ABSTRACT

Apparatus for reducing shock and overpressure particularly useful in connection with the sequential detonation of a series of nuclear explosives underground. A coupling and decoupling arrangement between adjacent nuclear explosives in the tubing string utilized to emplace the explosives is able to support lower elements on the string but yields in a manner which absorbs energy when subjected to the shock wave produced upon detonation of one of the explosives. Overpressure is accommodated by an arrangement in the string which provides an additional space into which the pressurized material can expand at a predetermined overpressure.

United States Patent [1 1 Walter [4 Oct. 21, 1975 APPARATUS FOR REDUCING SHOCK AND OVERPRESSURE [75] Inventor: Carl E. Walter, Pleasanton, Calif.

[73] Assignee: The United States of America as represented by the United States Energy Research and Development Administration, Washington, DC.

[22] Filed: Sept. 17, 1973 [21] Appl. No.: 398,262

[44] Published under the Trial Voluntary Protest Program on January 28, 1975 as document no. B 398,262.

[52] US. Cl l02/21.6; 102/26 [51] Int. Cl. F42B 3/02 [58] Field of Search 102/2l.4, 21.6, 21.8, 26,

[56] References Cited UNITED STATES PATENTS 137,196 3/1873 Gotham 102/26 Fox 102/21.8

l-leckman 102/21.4

[57] ABSTRACT Apparatus for reducing shock and overpressure particularly useful in connection with the sequential detonation of a series of nuclear explosives underground. A coupling and decoupling arrangement between adjacent nuclear explosives in the tubing string utilized to emplace the explosives is able to support lower elements on the string but yields in a manner which absorbs energy when subjected to the shock wave produced upon detonation of one of the explosives. Overpressure is accommodated by an arrangement in the string which provides an additional space into which the pressurized material can expand at a predetermined overpressure.

10 Claims, 4 Drawing Figures US. Patent Oct. 21, 1975 3,913,481

PRESSURE D/STA/VCE APPARATUS FOR REDUCING SHOCK AND OVERPRESSURE BACKGROUND OF THE INVENTION The invention described herein was made in the course of or under Contract No. W-7405-Eng-48 with the U.S. Atomic Energy Commission.

The invention relates to the detonation of explosives underground and more particularly to apparatus for reducing shock and overpressure resulting from the detonation of nuclear explosives underground.

The development and/or enhancement of the production of resources such as minerals, geothermal energy, etc., from subterranean reservoirs by the underground detonation of nuclear explosives, as well as the many other applications of such underground detonations, are now well known. In U.S. Pat. No. 3,688,843 entitled Nuclear Explosive Method for Stimulating Hydrocarbon Production from Petroliferous Formations," to Milo D. Nordyke, assignor to the United States of America as represented by the U.S. Atomic Energy Commission, the advantages of sequential detonation of a series of vertically spaced apart nuclear explosives is described in some detail. It will be appreciated that the detonation of an individual explosive in a particular series of emplaced explosives could subject other explosives of the series and/or the hardware associated therewith to sufficient shock and/or overpressures as to render them inoperable.

Accordingly, it is an object of this invention to provide apparatus which reduces the shock forces transmitted through the emplacement string upon the detonation of explosives underground. It is an additional object of the invention to provide apparatus that reduces overpressure transmitted through the formation and the stemming materials in the borehole casing within which the emplacement string is placed.

SUMMARY OF THE INVENTION Briefly summarized, the above and additional objects with respect to shock are accomplished by coupling and decoupling apparatus positioned within the emplacement string between the explosive to be detonated and the location along the emplacement string for which protection from shock is desired. With respect to the limitation of pressure, a section of the emplacement string between the explosive to be detonated and the location to be protected incorporates apparatus that provides an additional space into which the pressurized material can expand at a predetermined overpressure in order to dissipate the overpressure prior to its reaching the particular location to be protected. Preferably, the apparatus for limiting the overpressure is positioned between the coupling and decoupling apparatus and the location to be protected since some contribution to the overpressure may be contributed by operation of the coupling and decoupling apparatus.

Additional objects and advantages-and a better understanding of the invention will be apparent after consideration of the following description of a preferred embodiment thereof.

FIG. 2 is an enlarged view of portion 22 of FIG. 1, partially in section, illustrating preferred embodiment for decoupling shock,

FIG. 3 is an enlarged view of portion 3-3 of FIG. 1, at a smaller scale than FIG. 2, showing a preferred embodiment for dissipating overpressure, and

FIG. 4 is a graphical representation of pressure dissipation versus distance with and without the use of the embodiment of FIG. 3.

DESCRIPTION OF PREFERRED EMBODIMENT Referring now to FIG. 1, three nuclear explosive packages l2, l4 and 16 are shown suspended within casing 18 of drill hole 22 as a part of emplacement string 24. Shock brakes 26 and 28, which provide the coupling and decoupling function, and pressure limiter sections 32 and 34 are provided in the tubing string elements making up emplacement string 24 between nuclear expiosivc packages 12 and 14, and 14 and 16, respectively. The components making up shock brakes 26 and 28 are shown in greater detail in FIG. 2 and the components making up pressure limiter sections 32 and 34 are shown in FIG. 3.

As shown in FIG. 2, the shock brakes include two major components, lower member 36 and upper member 38. Relatively massive metal body 42 of lower member 36 terminates in a radially outwardly and downwardly tapered surface 44. A central cylindrical spacer 46 extends upwardly from surface 44 and a radially outwardly protruding flange 48 of larger diameter is fastened to the top thereof by complementary screw threads or other conventional means.

Upper member 38 includes a plurality of downwardly extending fingers 52, each of which has a bottommost surface 54 which is tapered complementarily to surface 44 of lower member 36 for a reason that will presently be explained. The lower part of fingers 52 extends radially inwardly toward cylindrical spacer 46 to provide horizontal surface 56 which underlies lower surface 58 of flange 48. It can be seen then, that the weight of lower member 36 and the portion of emplacement string 24 beneath it is transmitted to upper member 38 through cooperative surfaces 56 and 58 of fingers 52 and flange 48, respectively, when emplacement string 24 is suspended in the hole. It will be appreciated that the assembly of flange 48 onto spacer 46 will be accomplished after the alignment of members 36 and 38 in the relative positions illustrated in FIG. 2, and that the inner, vertically extending surface 60 of upper member 38 would provide sufficient clearance for the passage of flange 48 therethrough.

The function of shock brake 26 upon detonation of nuclear explosive 12 as the first of a series will now be explained. The shock wave generated by an explosion propagates through surrounding media at speeds proportional to the speeds of sound through those media.

The speed of sound through metals is faster than through the stemming media usually drilling mud, not shown in FIGS. 1 and 2, which fills the annulus between casing 18 and emplacement string 24. Therefore, the shock wave produced by detonation of explosive 12 will proceed toward explosive package 14 most rapidly through the emplacement string 24. Accordingly, it is the function of shock brake 26 to prevent the full intensity of the shock wave transmitted through emplacement string 24 from reaching nuclear explosive package 14 with disabling effects The forceable upward movement of lower member 36 of shock brake 12 caused by progression of the shock wave therethrough is resisted by the weight of emplacement string 24 above shock brake 26. The application of that upward force to upper member 38 is through fingers 52. However, the cross-section of the fingers and their ductility in conjunction with complementary tapered surfaces 44 and 54 are selected to effect an accommodation of that upward movement of lower member 36 through the radially outward and upward deformation of fingers 52. The thickness of the fingers and the radial dimension of the annulus between the fingers and casing 18 are selected so that the fingers are effectively swaged against the casing as their deformation proceeds. The deformation and swaging of fingers 52 against casing 18 not only provides for dissipation of energy from the shock wave, it also provides a barrier which seals casing 18 from the chimney that will be formed below it pursuant to the well-known chain of events following detonation of a nuclear explosive underground. Accordingly, stemming materials above shock brake 26 are thereby prevented from falling into the chimney and gases are prevented from passing out of the chimney upwardly to the surface.

It can be seen then that shock brake 26 performs three functions:

1. It arrests the motion imparted to the emplacement string by shock,

2. [t dissipates shock energy transmitted through the emplacement string through friction and deformation, and

3. [t closes the borehole off thereby minimizing the leakage of stemming material down into the cavity formed by the explosive and minimizing the flow of high pressure gases, i.e., reservoir gases and explosion generated gases, to the next adjacent explosive package in the series and, therefore, the surface of the earth.

It will be appreciated that a plurality of shock brakes can be utilized in a section of the emplacement string between any two nuclear explosives in order to provide redundancy or to optimize the design by dissipating the totol shock energy in several increments rather than one. Also, the function of shock brake 28 upon detonation of explosive 14 will be identical to that of shock brake 26, just described, upon detonation of explosive 12.

ln addition to the protection provided explosive package 14 from the shock wave produced by detonation of explosive 12 by shock brake 26, just described, it is also desirable to limit the pressure to which the explosure package 14 is subjected. This pressure comes from three sources. First, action of the shock brake may permit sufficient vertical movement of the emplacement string to compress the stemming material which surrounds it by a piston-like action. Second, casing 18 will be exposed to a high degree of compression from the outside when the shock wave traveling through the adjacent media passes the brake location (this is referred to as the bearhug" effect). Third, a compressive shock wave will be transmitted directly through the stemming material.

Accordingly, pressure limiting devices are incorporated into emplacement string 24 to counteract the pressure rise due to these causes. As shown in FIGS. 1 and 3, at least a portion 32 and a portion 34 of emplacement string 24 above shock brakes 26 and 28 are made up of a pipe section 61 which, while strong enough to support the weight of the emplacement string below, is designed to circumferentially collapse radially inwardly upon itself when subjected to predetermined overpressure. It is preferably made of a ductile material, such as aluminum, and preferably has a circumferential configuration, such as of a polygon with concave sides, which facilitates its circumferential collapse. The interior 62 of pressure limiter 32 is either maintained at a vacuum, or contains a material, such as air or a gas at low pressure, a foam, etc., which readily compresses.

The length, cross-sectional area and collapse strength of the pressure limiter can be selected to provide that volume necessary to keep the final pressure below a desired limit. Of course the selected collapse strength would be a value safely above normal emplacement pressure. For instance, if the maximum emplacement pressure were 5,000 psi, the collapse pressure would be chosen to fall between say, 8,000 and 12,000 psi.

FIG. 4 graphically illustrates the pressure dissipation with respect to distance with and without the use of a pressure limiter. Curve 64 represents the normal relationship and curve 66 the relationship with the use of a pressure limiter such as that just described. As shown by the graph, the use of the pressure limiter achieves a much more rapid decrease in pressure with distance until that selected pressure 68 at which pressure limiter section 32 begins to collapse upon itself is reached.

Accordingly, the shock brake and pressure limiting apparatus just described provide for preserving theintegrity and operability of nuclear explosive packages against the effects of shock and overpressure in instances where sequential firing of nuclear explosives is desired. It will be appreciated that both of these are forgiving designs in that uncertainties in shock intensity and pressure buildup may be easily accommodated by the addition to the shock attenuation and/or pressure limiting capacity by, for example, providing a plurality of these devices in a particular section of the emplacement string or by the particular selection of the parameters of those apparatus, or both. Those skilled in the art will appreciate that while the fundamental novel features of the invention have been shown and described and pointed out as applied to particular embodiments by way of example, various omissions, substitutions and changes may be made within the principle and scope of the invention as expressed in the appended claims.

What 1 claim is:

1. In a system for the production of underground resources by the sequential detonation of a plurality of spaced apart nuclear explosives therein, a combination of elements in an emplacement string for connecting together a pair of adjacent nuclear explosives for emplacement of the explosives into a borehole comprising:

a pair of tubing string elements between said pair of adjacent nuclear explosives,

means for coupling together said pair of tubing string elements whereby all emplacement string elements including nuclear explosives below the upper of said pair of elements are suspended from said upper element, and

means for decoupling said pair of tubing string elements in response to the shock wave produced by detonation of one of said pair of nuclear explosives and traveling through that one of said pair of elements adjacent to said one explosive whereby the shock wave transmitted to the other of said nuclear explosives through the other of said pair of elements is attenuated.

2. The combination of claim 1 wherein said decoupling means include a plurality of deformable members on one of said pair of tubing string elements and means on the other of said pair of elements for outwardly deforming said deformable members in response to forceable movement of one of said pair of elements toward the other.

3. The combination of claim 2 wherein said deformable members are a plurality of longitudinally extending finger-like members.

4. The combination of claim 3 wherein the thickness of said fingers, their ductility, and their outer circumferential dimensions are selected to facilitate the swaging of said fingers against the wall of a bore hole of predetermined size upon said forceable movement.

5. The combination of claim 4 wherein said coupling means comprise cooperative means on said fingers and said other member.

6. The combination of claim 1 including means for relieving the pressure built up in the stemming material surrounding said tubing string elements by the detonation of said one nuclear explosive.

7. The combination of claim 6 wherein said pressure relieving means comprise a longitudinally extending circumferentially compressible tube.

8. The combination of claim 7 wherein the interior of said compressible tube is maintained under a vacuum.

9. The combination of claim 7 wherein the interior of said compressible tube contains a compressible substance.

10. The combination of claim 7 wherein said decoupling means includes a plurality of longitudinally extending deformable fingers on one of said pair of tubing string elements and a tapered surface on the other of said pair of elements which spreads said fingers outwardly in response to forceable movement of one of said pair of elements toward the other, said coupling means comprise cooperative means on said fingers and said other members, and the thickness of said fingers, their ductility and their outer circumferential dimensions are selected to facilitate the swaging of said fingers against the wall of a bore hole of predetermined size upon said forceable movement. 

1. In a system for the production of underground resources by the sequential detonation of a plurality of spaced apart nuclear explosives therein, a combination of elements in an emplacement string for connecting together a pair of adjacent nuclear explosives for emplacement of the explosives into a borehole comprising: a pair of tubing string elements between said pair of adjacent nuclear explosives, means for coupling together said pair of tubing string elements whereby all emplacement string elements including nuclear explosives below the upper of said pair of elements are suspended from said upper element, and means for decoupling said pair of tubing string elements in response to the shock wave produced by detonation of one of said pair of nuclear explosives and traveling through that one of said pair of elements adjacent to said one explosive whereby the shock wave transmitted to the other of said nuclear explosives through the other of said pair of elements is attenuated.
 2. The combination of claim 1 wherein said decoupling means include a plurality of deformable members on one of said pair of tubing string elements and means on the other of said pair of elements for outwardly deforming said deformable members in response to forceable movement of one of said pair of elements toward the other.
 3. The combination of claim 2 wherein said deformable members are a plurality of longitudinally extending finger-like members.
 4. The combination of claim 3 wherein the thickness of said fingers, their ductility, and their outer circumferential dimensions are selected to facilitate the swaging of said fingers against the wall of a bore hole of predetermined size upon said forceable movement.
 5. The combination of claim 4 wherein said coupling means comprise cooperative means on said fingers and said other member.
 6. The combination of claim 1 including means for relieving the pressure built up in the stemming material surrounding said tubing string elements by the detonation of said one nuclear explosive.
 7. The combination of claim 6 wherein said pressure relieving means comprise a longitudinally extending circumferentially compressible tube.
 8. The combination of claim 7 wherein the interior of said compressible tube is maintained under a vacuUm.
 9. The combination of claim 7 wherein the interior of said compressible tube contains a compressible substance.
 10. The combination of claim 7 wherein said decoupling means includes a plurality of longitudinally extending deformable fingers on one of said pair of tubing string elements and a tapered surface on the other of said pair of elements which spreads said fingers outwardly in response to forceable movement of one of said pair of elements toward the other, said coupling means comprise cooperative means on said fingers and said other members, and the thickness of said fingers, their ductility and their outer circumferential dimensions are selected to facilitate the swaging of said fingers against the wall of a bore hole of predetermined size upon said forceable movement. 